Make "bridge-by-value types are not supported in struct fields" more specific

docs/errors/bridge_field.md

@@ -0,0 +1,65 @@
+<!-- <internal link> -->
+
+# Bridge types as struct fields
+
+## Overview
+
+Crubit does not support exposing struct or union fields with bridge types.
+
+Bridge types are types that are converted at runtime between C++ and Rust
+(`std::optional` mapping to `Option`), but have different underlying
+representations. (See crubit.rs/types)
+
+If a field is a bridge type, this conversion is impossible: field accesses do
+not run any conversion logic, and the field is required to have identical
+representation across both languages. As a result, the field will not have the
+bridge type conversion applied, but instead will be replaced by an opaque blob
+of bytes.
+
+## Example
+
+Consider the following C++ struct:
+
+```c++
+#include <optional>
+#include <cstdint>
+
+struct Config {
+  std::optional<int> foo;
+};
+```
+
+Crubit will treat the `foo` field of `Config` as an opaque blob of bytes that
+cannot be accessed directly from Rust.
+
+## Workaround: Add getter and setter methods {#workaround}
+
+To access or modify the field from Rust, you can add getter and setter methods.
+Crubit *can* generate bindings for functions that accept or return bridge types
+by value.
+
+For example:
+
+```c++
+#include <optional>
+
+struct Config {
+  std::optional<int> foo;
+
+  // Getter returning by value (mapped to Option<i32> in Rust)
+  std::optional<int> get_foo() const { return foo; }
+
+  // Setter accepting by value
+  void set_foo(std::optional<int> val) { foo = val; }
+};
+```
+
+Then in Rust, you can use these methods:
+
+```rust
+let mut config = Config::default();
+config.set_foo(Some(42));
+assert_eq!(config.get_foo(), Some(42));
+```
+
+See crubit.rs/cpp/best_practices#bridging for more information.

rs_bindings_from_cc/generate_bindings/generate_struct_and_union.rs

@@ -135,7 +135,8 @@ fn get_field_rs_type_kind_for_layout(
     }
 
     if type_kind.is_bridge_type() {
-        bail!("Bridge-by-value types are not supported in struct fields.")
+        bail!("crubit.rs/errors/bridge_field: '{}' is a bridge type, but fields must be layout compatible between Rust and C++.",
+            type_kind.display(db))
     }
 
     for target in

rs_bindings_from_cc/test/golden/composable_bridging.h

@@ -19,6 +19,10 @@ struct
     // clang-format on
     CppStruct {};
 
+struct StructWithBridgeField {
+  CppStruct bridge_field;
+};
+
 CppStruct ReturnCppStruct();
 
 void TakeCppStruct(CppStruct);

rs_bindings_from_cc/test/golden/composable_bridging_rs_api.rs

@@ -18,6 +18,32 @@
 // order for the generated code to properly compile. This example just serves to
 // illustrate what the generated code will look like.
 
+#[derive(Clone, Copy, ::ctor::MoveAndAssignViaCopy)]
+#[repr(C)]
+///CRUBIT_ANNOTATE: cpp_type=StructWithBridgeField
+pub struct StructWithBridgeField {
+    /// Reason for representing this field as a blob of bytes:
+    /// crubit.rs/errors/bridge_field: 'crate::RustStruct' is a bridge type, but fields must be layout compatible between Rust and C++.
+    pub(crate) bridge_field: [::core::mem::MaybeUninit<u8>; 1],
+}
+impl !Send for StructWithBridgeField {}
+impl !Sync for StructWithBridgeField {}
+unsafe impl ::cxx::ExternType for StructWithBridgeField {
+    type Id = ::cxx::type_id!("StructWithBridgeField");
+    type Kind = ::cxx::kind::Trivial;
+}
+
+impl Default for StructWithBridgeField {
+    #[inline(always)]
+    fn default() -> Self {
+        let mut tmp = ::core::mem::MaybeUninit::<Self>::zeroed();
+        unsafe {
+            crate::detail::__rust_thunk___ZN21StructWithBridgeFieldC1Ev(&raw mut tmp as *mut _);
+            tmp.assume_init()
+        }
+    }
+}
+
 #[inline(always)]
 pub fn ReturnCppStruct() -> crate::RustStruct {
     unsafe {
@@ -568,6 +594,9 @@ mod detail {
     #[allow(unused_imports)]
     use super::*;
     unsafe extern "C" {
+        pub(crate) unsafe fn __rust_thunk___ZN21StructWithBridgeFieldC1Ev(
+            __this: *mut ::core::ffi::c_void,
+        );
         pub(crate) unsafe fn __rust_thunk___Z15ReturnCppStructv(
             __return_abi_buffer: *mut ::core::ffi::c_uchar,
         );
@@ -617,6 +646,11 @@ mod detail {
 }
 
 const _: () = {
+    assert!(::core::mem::size_of::<crate::StructWithBridgeField>() == 1);
+    assert!(::core::mem::align_of::<crate::StructWithBridgeField>() == 1);
+    static_assertions::assert_impl_all!(crate::StructWithBridgeField: Copy,Clone);
+    static_assertions::assert_not_impl_any!(crate::StructWithBridgeField: Drop);
+    assert!(::core::mem::offset_of!(crate::StructWithBridgeField, bridge_field) == 0);
     assert!(::core::mem::size_of::<crate::Vec3>() == 12);
     assert!(::core::mem::align_of::<crate::Vec3>() == 4);
     static_assertions::assert_impl_all!(crate::Vec3: Copy,Clone);

rs_bindings_from_cc/test/golden/composable_bridging_rs_api_impl.cc

@@ -20,6 +20,16 @@
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wthread-safety-analysis"
 
+static_assert(sizeof(struct StructWithBridgeField) == 1);
+static_assert(alignof(struct StructWithBridgeField) == 1);
+static_assert(CRUBIT_OFFSET_OF(bridge_field, struct StructWithBridgeField) ==
+              0);
+
+extern "C" void __rust_thunk___ZN21StructWithBridgeFieldC1Ev(
+    struct StructWithBridgeField* __this) {
+  crubit::construct_at(__this);
+}
+
 extern "C" void __rust_thunk___Z15ReturnCppStructv(
     unsigned char* __return_abi_buffer) {
   ::crubit::Encoder __return_encoder(::crubit::CppStructAbi::kSize,